Fine particles on which various kinds of biological substances are immobilized have been widely utilized in medical diagnosis, a biotechnology research field and the like. Fine particles that are formed of a variety of materials and have various particle diameters, forms and functions have been studied and proposed therein. Among them, there exists a technology in which magnetic particles are utilized. Specific examples include a technology in which magnetic particles on which an antibody is immobilized are used to perform affinity column chromatography. At this time, in latex beads, an antigen is purified by repeating centrifugal separation and washing operation. Therefore, a large amount of solvent is required. On the other hand, when the magnetic particles are used, the complicated operation as described above is not required because particles can be easily separated by means of a magnet. That is, in comparison with the latex beads having no magnetism, the magnetic particles have an advantage of capability of rapidly and simply purifying the antigen. Moreover, the magnetic particles are also adapted to automation using a machine because such centrifugal separation and application of the large amount of solvent become unnecessary. For the above reason, immunological magnetic particles have been frequently utilized as a carrier in immunodiagnosis (sandwich method) in recent years.
In order to improve molecular recognition properties of the magnetic particles, a surface area thereof is preferably increased. In this regard, in order to further increase the surface area per unit volume for magnetic beads, further reduction of a particle diameter is effective. On the other hand, if the particle diameter of the magnetic particles falls in a nanometer order, the magnetism of the particles is significantly weakened. Further, magnetic separation by means of the magnet becomes significantly difficult under the influence of Brownian motion in water. In order to solve this problem, proposals have been made on arts of using particles in which a thermoresponsive polymer is immobilized on a surface layer of magnetic nanoparticles having a particle diameter of several tens of nanometers (see Patent Literatures 1 and 2). A target substance is detected depending on a change of colors (into transparency) of magnetic materials by aggregating thermoresponsive magnetic particles by heating therein. Specifically, absorbance of a reaction liquid over time is monitored by heating the liquid after a reaction with the target substance, and existence of the target substance (analyte) is judged by the absorbance being gradually decreased.    Patent Literature 1: WO 2008/001868 pamphlet    Patent Literature 2: Japanese Patent No. 4518767